In an electrical power system, auto transformers are often used to connect two different voltage systems or buses in an electrical substation. The auto transformers used in such bus-tie connections represent the majority of transformers in an electrical power system. In a conventional design auto transformer for bus-tie applications only the reactive power flow through the transformer can be modified by the means of so called voltage regulating winding and associated voltage tap changer. The active power flow through the transformer which is determined by the connected power system characteristics and the transformer short-circuit impedance can not be modified to accommodate for future power system developments. As a result, power system operation of a conventional auto transformer can become very uneconomical with time.
In the past, there have been auto transformers for bus-tie applications able to modify the active power flow through the transformer. These auto transformers use a so called pseudo phase-angle regulating winding and associated tap changer. The term pseudo indicates that the phase-angle and active power flow regulation is not an independent regulation and is accompanied by a reactive power flow and low voltage bus regulation. The reactive power flow and associated low voltage bus regulation determined by the active power flow regulation can be in a desired or non desired direction.
The present invention uses an auto transformer design with two independent regulating windings. For any practical purposes, selecting a particular tap position on the two regulating windings, any desired combination of phase-angle (active power flow) and voltage-ratio (reactive power flow) can be obtained. The invention provides for independent active and reactive power flow controls, the most desirable operation feature of an auto transformer used in system interconnection or bus-tie applications.
In the majority of the electrical power systems, the phase-angle between the high voltage and a low voltage buses in a substation is less than 10 degrees. As a consequence, only a small change in the phase-angle between primary and secondary induced voltages of an auto transformer connecting such voltage buses would produce significant changes in the active power flow through the auto transformer.